Powder metallurgy method of forming an age hardenable ferrous alloy



July 28, 1970 S. W. M GEE 3,522,115 POWDER METALLURGY METHOD OF FORMING AN AGE HARDENABLE FERROUS ALLOY Filed Aug. 2, 1968 2 SheetsSheet 1 70 pi er W550 4r mm a 2- 35 i/Nfffffi 47 2200 F X All), E

Jdy 28, 1970 s. w. MOGEE ET AL 3,522,113

POWDER METALLURGY METHOD OF FORMING AN AGE HARDENABLE FERROUS ALLOY Filed Aug. 2, 1968 2 Sheets-Sheet 2 74/2 far/er ,lffwv/qyj.

United States Patent 3,522,115 POWDER METALLURGY METHOD OF FORMING AN AGE HARDENABLE FERROUS ALLOY Sherwood W. McGee, Lisle, and Eugene R. Andreotti,

Geneva, Ill., and Joel S. Hirschhorn and David A. Westphal, Madison, Wis., assignors to Burgess-Norton Mfg.

Co., Geneva, Ill., a corporation of Illinois Filed Aug. 2, 1968, Ser. No. 749,735 Int. Cl. C21d 1/00 U.S. Cl. 148-126 11 Claims ABSTRACT OF THE DISCLOSURE A powder metallurgy ferrous alloy of high physical properties and the method of making the alloy. In carrying out the method, iron powders and ferro alloy powders and additional metal powders selected for their strengthening effect are used. The ferro alloy powders comprise alloys of iron and metals selected for their strengthening effect. The powders are selected in accordance with suitable size relationships and are in general of very small dimensions. They are combined in proper order to insure a statistically random distribution of all the constituent powders. As a precaution to avoid premature agglomeration, the iron powder is preferably first added and this is followed by adding, in sequence, the most finely divided powders, that is to say the smallest constituents, singularly and blending each after such addition for a suflicient time to accomplish thorough mixing. The completely mixed powders are compacted under substantial pressure and the product is sintered. After sintering, the product is put through a thermal cycle selected to produce age hardenmg.

This invention relates to a sintered metal alloy and to the method of producing it. It has for one object to provide a method of forming a ferrous alloy article by powder metallurgy in which the powders are selected to produce a metal article having the desired physical prop erties after the completion of the method disclosed herein.

It has for another object to provide a method of pro ducing a ferrous metallic alloyed article by mixing iron powder, ferro alloy powders and powders having a strengthening effect on the completed article and treating a mixture of such powders according to a thermal cycle selected to produce age hardening.

Other objects will appear throughout the specification and claims.

In carrying out the method, the chosen powders are mixed in a manner to prevent premature agglomeration and to accomplish an essentially statistically random mixture. During the mixing process a solid lubricant of approximately 1% of the mass is added. Alternatively, the die walls may be separately lubricated and the 1% solid lubricant, therefore, omitted from the powders. Thereafter the mixture is compacted preferably in a suitable die assembly and at room temperature. It is compacted to achieve densities in excess of 90% of the theoretical density of the fully homogeneous alloy. To accomplish this, pressures of 30 to 60 tons per square inch may be employed.

Experience has shown that casual selection of the sequence of mixing is generally unsatisfactory in as much as combinations of only two or possibly three of the lighter constituents and the more finely divided constituents will form in preference to the statistically random distribution of all the constituents which is a desired end result. As a general precaution to avoid this happening, we find a useful practice consists of first adding to the mixing container a quantity of the heavy base iron of at least 30% of the total amount and, following this, adding 3,5ZZ,1 l5 Patented July 28, 1970 in sequence the most finely divided of the constituents singlely and blending each after such additions for a period of time to thoroughly mix these with the stated base quantity of iron before the next light constituent is added. In this way the undesired agglomeration of two or more additions is avoided.

Physical dimensions and specific gravity act together to control the order in which the materials and powders are introduced into the base mixture.

Our Work indicates that both the sintering temperature and sintering time can be usefully decreased as the particle size of the starting powders is decreased. This is substantiated by physical theory in as much as the free energy driving the sintering process is also proportioned to the free specific surface of the powder and, therefore, it is to be expected that more finely divided powders will sinter more rapidly at lower temperatures in contrast to coarser powders of the same chemistry. The trends indicated in our work give good reason to believe that the properties we teach for tensile strengths, hardness and elongation are within practical reach by controlling the average particle size and the total mixture including the iron constituent to not greater than ten microns and otherwise practicing the invention as disclosed except conducting the sintering process at temperatures on the order of 2100 F. in sintering times on the order of onehalf to one hour.

After the compaction, the article is ejected from the die and is sintered at a minimum temperature of the order of 2200 F This sintering is preferably carried out for 2 /2 hours or longer. Under certain circumstances suitable sintering may be accomplished in shorter times. Generally, after sintering the article is allowed to cool and is reheated to a temperature of the order of 15 00 F. Thereafter it may be quenched and finally reheated to the ageing temperature of the order of 900 F. At this temperature ageing is accomplished in from 2 to 4 hours. The article so formed and so thermally treated has been found in practice to produce mechanical properties of high quality such as a tensile strength of 120,000 pounds per square inch minimum with a useful ductility measurable as 2% in elongation in tensile testing and high hardness sufficient for wear resisting of the order of 50 to Rockwell A.

Articles produced by the method of this application have been found to comprise good hardness and tensile strength after relatively short periods of sintering, although longer periods of sintering somewhat enhance the physical properties.

The fact that these powders as individual particles are very finely divided but at the same time chemically different gives us a strong driving force for diffusion. Diffusion in itself consists of the migration of atoms through the solid crystal lattices of the metals involved and when it occurs accounts for the movement of atoms from one particle in the powder mixture to adjacent particles in the mixture and the reverse process wherein the donor particles also receive atoms from their adjacent particles with the end result being the development of new strongly bonded sintered crystals. Generally the diffusional movement of atoms proceeds faster between heterogeneous substances than between substances of the same kind.

FIG. 1 is a diagram showing the relationship between the time of sintering and the hardness produced. The lines on the graph of this figure indicate three different alloys which are described in detail herein.

FIG. 2 is a diagram illustrating the relationship between the sintering time and the tensile strength produced. The separate lines indicate the conditions produced in each of similar but slightly varying alloys.

Several examples of the method are given below. In general they have in common a number of features, each of which is applied to the treatment of the mass of metal powder which includes iron powder, ferro alloy powders and elemental powders. Iron forms the predominant constituent of the mass in each case and the ferro alloy constituents include pre-alloys of iron and nickel, molybdenum, cobalt, titanium and aluminum. In some of the examples given below all of these ferro alloy powders are included. In others, only nickel, titanium and aluminum are included.

These advantages are both physical and chemical. The physical advantages derive from the fact that in the dilute ferro alloy a greater number of particles are actually present than would be present if the simple titanium, aluminum or other non-ferrous constituents were present alone and, of course, with the greater number of particles contained in the ferro alloy form, the attainment of a uniform mixture becomes easier. The chemical advantages refer to the lesser degree of oxidation suffered by the non-ferrous constituent such as titanium or aluminum in its dilute form even under the best practical protective atmospheres.

A second advantage or reason in the chemical category is lowering the energy barrier required to initiate solution of the non-ferrous constituent such as titanium or aluminum in the greater body of the alloy. This does not contradict previous statements that heterogeneity is desirable to promote rapid diffusion in as much as a high degree of heteregeneity is still retained by the particles of ferro alloy and the main body of the alloy but what is achieved is an initial solution of these more difiicultly dissolved non-ferrous alloy constituents.

This alloy may be formed by the admixture of the following constituents to a basic high purity iron powder in the mesh size range of 80 and down. The following constituents are admixed:

Iron 50% aluminum pre-alloy of 325 mesh particle size 0.2%

Iron 26% titanium pre-alloy of 325 mesh particle size Cobalt of 400 mesh particle size 9% Molybdenum of up to 4 microns 5% Iron nickel pre-alloy of 325 mesh particle size These constituents are blended to a degree where an essentially statistical random mixture of the particles is attained and in such mixing process an amount of solid lubricant of approximately 1% is usually used. This mixture is next compacted in tool steel or carbide form dies at ordinary temperatures to achieve densities in excess of 90% the theoretical density of the fully homogeneous alloy. Compaction pressures on the order of 50 to tons per square inch are generally employed in such densification and in this process the admixed lubricant serves the purpose of maintaining separation between the powder mixture and the die wall surfaces. Upon ejection from the die the compacted body is next sintered at a minimum temperature of 2200 F. for 2 /2 hours or longer under suitable protective atmospheres, then is allowed to cool, is reheated to 1500 F., is quenched and is next aged at temperatures on the order of 900 F. to produce mechanical properties approximated by a tensile strength of 120,000 pounds per square inch minimum but up to and exceeding tensile strengths on the order of 200,000 pounds per square inch with useful ductility measurable as 2% elongation in tensile testing and high hardness sufilcient for wear resisting applications on the order of 50 to Rockwell A.

All of the examples have in common the fact that they include iron powder as more than 60% of their total mass. Each also includes a ferro alloy of a metal which has an important strengthening effect in the final article formed. The metal powders in each of the examples are 4 of extremely small dimensions. A further similarity between all of the examples is that preferably each is treated by the same heating cycle. The analysis of six examples of suitable powder groupings is set out below; all proportions to yield a final mixture comprising:

Mo: 3-4 microns Co: 400 mesh Fe26% Ti: 325 mesh Fe-50% Al: 325 mesh Fe: mesh ALLOY II Fe-36% Ni: 325 mesh Mo: 3-4 microns Co: 400 mesh Fe26% Ti: 325 mesh Fe50% A1: 325 mesh Fe: 325 mesh ALLOY III Fe36% Ni: 325 mesh Mo: 3-4 microns Co: 400 mesh Fe-26% Ti: 325 mesh Fe-50% Al: 325 mesh Fe: 1-9 microns carbonyl ALLOY IV Fe-36% Ni: 325 mesh Fe-50% M0: 325 mesh Fe-70% Co: 325 mesh Fe-26% Ti: 325 mesh Fe-50% A1: 325 mesh Fe: 100 mesh ALLOY V Fe36% Ni: 325 mesh Fe-50% M0: 325 mesh Fe-70% Co: 325 mesh Fe-26% Ti: 325 mesh Fe-50% A1: 325 mesh Fe: 325 mesh ALLOY VI Fe-36% Ni: 325 mesh Mo: 3-4 microns Fe-70% Co: 325 mesh Fe-25% Ti: 325 mesh Fe-50% A1: 325 mesh Fe: 100 mesh Example I A mixture of powders proportioned and described as that given for Alloy I was compacted in tooling as described under 60 tons per square inch pressure. The lubri cant in this instance was stearic acid. The compacted den sity was 7.28 grams per cc. or 91.0% theoretical. The following tensile strengths were obtained for specimens so compacted when sintered in argon with 5% hydrogen at 2350 F. for the periods of time described after age hardening.

Hours: P.s.i. 2 120,000 3 130,000 5 160,000 9 180,000

Example II Hours: P.s.i. 1 150,000 3 180,000 5 200,000 8 240,000

We claim:

1. The method of producing a ferrous alloy which comprises the steps of mixing metallic powders comprisferro nickel, molybdenum, cobalt, ferro titanium, ferro aluminum and iron powder,

compacting a mixed mass of such said powders, sintering the compacted mass for approximately /2 to 2 hours at a temperature of the order of 2100 F. under a protective atmosphere,

cooling the mass and, thereafter reheating the mass to a temperature of the order of further cooling the mass,

thereafter bringing the mass to a temperature of the order of 900 F. and

maintaining it at that temperature for a time sufficient to produce the required age hardening.

2. The method of claim 1 further characterized by the fact that the powders in the compacted mass are present in quantities to produce in the compacted mass approximately 3. The method of claim 1 further characterized by the fact that the ferro nickel powder is of 325 mesh,

the molybdenum is of 3 to 4 microns in size,

the cobalt powder is of 400 mesh,

the ferro titanium is of -325 mesh,

the ferro aluminum is of 325 mesh.

4. The method of claim 1 further characterized by the fact that the ferro nickel powder is of 325 mesh,

the molybdenum is of 3 to 4 microns in size,

the cobalt powder is of -400 mesh,

the ferro titanium powder is of 325 mesh,

the ferro aluminum is of 325 mesh,

the iron powder is of -325 mesh.

5. The method of claim 1 further characterized by the fact that the ferro nickel powder is of 325 mesh,

the molybdenum powder is of 3 to 4 microns in size,

the cobalt powder is of 400 mesh,

the ferro titanium powder is of 325 mesh,

the ferro aluminum powder is of -325 mesh,

the iron powder is of 10 imicrons or finer in size.

6. The method of claim 1 further characterized by the fact that the ferro nickel powder is of 325 mesh,

the molybdenum powder is of 3 to 4 microns in size,

the ferro cobalt powder is of 325 mesh,

the ferro titanium powder is of -325 mesh,

the ferro aluminum powder is of '325 mesh,

the iron powder is of 10 microns or finer in size.

7. The method of claim 1 further characterized by the fact that the iron nickel powder is of 325 mesh,

and that there is included ferro molybdenum powder of 325 mesh,

ferro cobalt powder of 325 mesh,-

ferro titanium powder of -325 mesh, and

ferro aluminum powder of -325 mesh.

8. The method of claim 3 further characterized by the fact that in the ferro nickel powder, nickel comprises 36%,

in the ferro titanium powder, titanium comprises 26%,

in the ferro aluminum powder, aluminum comprises 9. The method of producing a high strength ferrous base powder alloy which includes mixing a relatively large proportion of powdered iron and a relatively smaller proportion of hard metal powder and of ferro alloy powders in which the alloy is a hard metal,

compacting the materials into a cohesive mass,

sintering the mass in a protective atmosphere,

cooling in a manner to produce a martensitic phase substantially free of internal stresses and thereafter,

age hardening it through reactions occurring within the martensitic phase contained in the sintered alloy mass.

10. The method of claim 9 characterized by the fact that the hard metal powders include metals selected for their strengthening effect on the final alloy produced by the method, some unalloyed and some being alloyed with iron.

11. The method of producing a high strength ferrous base powder alloy which includes mixing a relatively large proportion of powdered iron and a relatively smaller proportion of hard metal powder and of ferro alloy powders in which the metal alloyed with the iron is a hard metal, the hard metals being selected from the fourth, fifth or sixth periods of the periodic chart of elements,

then compacting the powdered materials into a cohesive mass,

sintering the mass in a protective atmosphere,

cooling it at a non-stressing rate of cooling in a manner to produce martensitic phase and thereafter,

age hardening the sintered alloyed mass.

References Cited UNITED STATES PATENTS 2,828,202 3/1958 Goetzel 148126 X 3,053,706 9/1962 Gregory 148126 X 3,303,066 2/1967 McGee 148-126 X FOREIGN PATENTS 390,426 4/1933 Great Britain.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant Examiner US. Cl. X.R. 

